Communication using multiple antennas is an efficient method for performing high-speed data transmission and for increasing the capacity of a communication system. However, it is not easy to apply multiple antennas to a mobile station because of hardware space restrictions. As a result, base station typically uses multiple antennas and a mobile station employs beamforming (BF) using a single antenna.
FIG. 1 illustrates conventional downlink beamforming (DL-BF) that does not consider interference cancellation.
Referring to FIG. 1, base stations 101, 102 and 103 transmit a signal using maximum power while considering only mobile stations 110, 120, and 130 located in sectors of the base stations 101-103, respectively. In other words, each of the base stations 101-103 does not consider an interference signal introduced to its neighboring sector. While such a signal transmission method is the best scheme for a single-cell structure, it may degrade the overall system performance in a multi-cell environment due to interference between cells or sectors.
To solve the problem, a downlink beamforming (DL-BF) method in which an uplink (UL) weight vector is also applied to a downlink (DL) operation may be used. The DL-BF method can be implemented on the assumption that an UL channel state and a DL channel state are similar to each other.
When a beam pattern used in an UL is applied to a DL, performance degradation does not occur if a reception signal and an interference signal are received at different angles in space. This is the case with signal transmission. However, in an actual channel environment, the reception signal and the interference signal are not likely to be separated completely in space. Moreover, since the link characteristics of the DL and the UL (e.g., noise variances) are different from each other, the use of an uplink beamforming (UL-BF) weight vector for a DL-BF weight vector degrades system performance. Therefore, there is a need for a new interference cancellation method capable of using a DL-BF weight vector.